fix(nested-clients): propagate clientConfig to inner STS client in role assumption

[kuhe]

Issue

#7439

Description

propagate clientConfig to inner STS client

Testing

• new integration tests
• CI

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts).
• If you wrote E2E tests, are they resilient to concurrent I/O?
• If adding new public functions, did you add the @public tag and enable doc generation on the package?

[jackrein]

Will it be necessary to set both the parent client and the credentials provider clientConfig? The comment in the docs states that region and profile will be propagated, but nothing else.

new S3Client({
    customUserAgent: [["custom-user-agent", "1.0.0"]],
    credentials: fromNodeProviderChain({
        clientConfig: {
            customUserAgent: [["custom-user-agent", "1.0.0"]],
        }
    })
});

[kuhe]

Yes, setting a user agent value on the both outer and inner client configs will be required even after this change is published.